Incident Photosynthetically Active Radiation Research Articles

Characterization of light intensity and quality, vegetative, flowering and fruiting traits in high and super-high density olive hedgerows

Orchard design (intra- and inter-row distance) defines the space allotted to each tree and the light environment for growth in olive hedgerows. Shading between neighboring trees affects the light intensity and quality, modifying the tree vegetative, flowering and fruiting characteristics. In this study, the incident photosynthetically active radiation (PAR) was simulated and the red-to-far-red ratio (R/FR) reflected by neighboring hedgerows down the canopy walls was measured. An analysis is presented of the response of olive vegetative, flowering, fruiting and productive traits to hedgerows of high (HD) and super-high density (SHD) orchards. The study was carried out during the 2018–2019 and 2019–2020 seasons in two 10-year-old olive cv. Genovesa orchards, one in HD (7 x 3.5 m) and the other in SHD (4 x 1.5 m). In both systems, continuous rows were used for measurements of light environment and vegetative, flowering and fruiting characteristics. The R/FR ratio and mean daily horizontal incident PAR were significantly higher in HD than in SHD. One-year-old shoots of HD hedgerows had shorter internodes in L position than U, M and L positions of SHD. Inflorescence number per shoot in the M and L positions of HD were triple than those of corresponding positions of SHD hedgerows. The mean yield per ha for both seasons was similar between HD and SHD hedgerows (average 9.3 and 9.4 t/ha, respectively). A greater planting density in olive hedgerows reduces the R/FR ratio reflected by neighboring trees while reducing incident PAR with increases in the hedgerow height. As a result, more illuminate HD hedgerows have greater specific leaf mass, higher leaf area density and higher axis-order angle compared to SHD hedgerows. This study seeks a new way to understand and measure the suitability of an olive cultivar trained in hedgerows at different planting densities.

Crop productivity estimation by integrating multisensor satellite, in situ, and eddy covariance data into efficiency-based model.

Accurate and quantitative regional estimates of the carbon budget require an integration of eddy covariance (EC) flux-tower observations and remote sensing in ecosystem models. In this study, a simple remote sensing driven light use efficiency (LUE) model was used to estimate the primary productivity for major cropping systems using multi-temporal satellite data over the Saharanpur district in India.The model is based on radiation absorption and its conversion into biomass. The LUE model was implemented for major crop rotations derived from the time-series of Sentinel-2 and Landsat 8 with monthly satellite-based spatially explicit fields of photosynthetically active radiation (PAR), fraction of absorbed PAR (fAPAR) and down-regulated light use efficiency. Incident PAR and fAPAR were estimated on monthly basis from the ground-calibrated empirical equation using INSAT-3D insolation product and remote sensing-based vegetation indices, respectively. Spatial LUE maps created by down-regulating maximum LUE (EC tower-based) with water and temperature stressors derived from land surface water index (LSWI) and EC-based cardinal temperature, respectively. LUE-based modeled GPP over the sugarcane-wheat system was found higher than the rice-wheat system in Saharanpur district. This is because C4 crop (sugarcane) has very high photosynthetic efficiency compared to C3 crops (rice and wheat). Modeled GPP over the sugarcane-wheat system was found in good agreement with observed EC tower-based GPP (Index of Agreement = 0.93). Further regionally calibrated remote sensing-based LUE model well captures gross photosynthesis rates (GPP) over cropland ecosystem compared to globally modeled MODIS GPP product.

Shading and PAR under different density agrivoltaic systems, their simulation and effect on wheat productivity

The introduction of renewable energy to mitigate climate change and the need for sufficient land to increase food production are mutually exclusive. Agrivoltaic systems (AVS) integrate the production of agricultural crops and electric power on the same piece of land but the main issue is the reduced availability of light to crops below the AVS, which restricts optimum crop growth and yield. This study examines the radiation and shade distribution over the crop surface among three densities of photovoltaic (PV) panels {Partial density (PD), Half density (HD) and Full density (FD)} under the AVS. Wheat variety GW 496 was chosen to grow under the AVS with line sowing and drip irrigation. Among three densities of photovoltaic (PV) panels, the proportion of shaded area over the crop surface was found highest in full density plot and lowest in partial density plot. The shaded area under AVS varied from 24.1% to 75.4% of the total area. The shaded area under different density plots, adversely affects the availability of photosynthetically active radiation (PAR) over the crop surface. The reduction of PAR under the AVS was more affected by panel density. Maximum reduction in PAR under AVS was observed in FD plot and minimum in the PD plot. A sharp increase in the PAR level was observed from shading to open sun conditions. The lowest value of PAR level (56 μmol m−2 s1) was recorded in just below the panel’s array. The consistent overestimation of PAR was observed during the crop growing period in all the PVPs density plots. The result revealed that the grid points under shade conditions showed a better association between observed and simulated incident PAR than unshaded grid points. Furthermore, the simulation results revealed that the model modeled PAR better in PD plot. The study also found that wheat grown under no shade yielded 4.51 t/ha over reduced light either in PD (3.06 t/ha), HD (2.77 t/ha), and FD plots (2.62 t/ha). Shading treatments showed that the crop yield decreased with an increase in shading intensity.

Light interception and radiation use efficiency in quinoa (Chenopodium quinoa Willd.)

AbstractQuinoa (Chenopodium quinoa Willd.) is an annual plant belonging to the Amaranthaceae family. The total accumulated incident photosynthetic active radiation (PAR) in a plant is a function of the site, year, sowing date, and phenology, whereas the amount of PAR intercepted by the plant is dependent on its leaf area index (LAI) and light extinction coefficient (KPAR). This study investigated changes in KPAR and radiation use efficiency (RUE) levels in quinoa before and after inflorescence (fruit set) color change under various temperature and light conditions through the sowing date. This study was conducted in the Jihad Keshavarzi allocated field of Khalilabad city in Khorasan Razavi province, Iran, using a randomized complete block design with three replications over two seasons, 2017–2018 and 2018–2019. Treatments included eight and seven sowing dates in the first and second season years, respectively. The results showed that the mean of KPAR was 0.66 and 0.31 in the first and second years, respectively. Also RUE in vegetative stages of development was higher than during the grain‐filling period. The mean of RUE was 2.60 ± 0.06 and 0.78 ± 0.17g/MJ in the first year before and after changing the color of the inflorescence (fruit set), respectively; these values were 2.12 ± 0.04 and 1.01 ± 0.61 g/MJ in the second year. The results indicated the base (TBRUE), lower optimum (TP1RUE), upper optimum, and ceiling temperatures of quinoa for RUE were determined to be 3, 18.8, 25.5, and 35°C, respectively.

Multi-site assessment of the potential of fine resolution red-edge vegetation indices for estimating gross primary production

Gross primary production (GPP) models driven by fine resolution remote sensing data characterize the spatial and temporal heterogeneities in plant photosynthesis, which is largely dependent on biome-specific maximum photosynthetic capacity. The red-edge reflectance, sensitive to leaf chlorophyll content, is a good proxy of maximum photosynthetic capacity. More importantly, studies show that the red-edge reflectance-related chlorophyll content index (CIr) multiplied by the incident photosynthetic active radiation (PARin) strongly correlates to GPP estimated at carbon flux towers (GPPflux). Yet, to the best of our knowledge, there is no systematic study investigating the general relationship between fine spatial resolution CIr and GPP among biomes and the relationship between CIr and maximum photosynthetic capacity in GPP models. To provide an overview on incorporating space-borne CIr into a GPP model, we applied fine resolution Sentinel-2-derived CIr and GPPflux over 57 flux sites representative of 10 biomes. We investigated the relationship between CIr and GPPflux, and the spatio-temporal relationship between CIr and ecosystem maximum photosynthetic capacity indicated by the potential ecosystem light use efficiency (LUEpot). We also evaluated the relationship between other five vegetation indices (VIs) and GPPflux. Results showed that the CIr multiplied by PARin has a higher agreement (R2 > 0.5) with GPP than other VIs. A universal relationship exists between the CIr multiplied by PARin and GPP, except for forest biomes. The CIr also strongly (R2 > 0.5) relates to the LUEpot during the peak of the growing season. The CIr has a low spatial variance (CV = 0.25) among biomes, highlighting that CIr can be a proxy of maximum photosynthetic capacity in GPP models that do not require biome-dependent coefficients. This study provides insight for incorporating CIr into GPP models and better quantifying global terrestrial photosynthesis.

Implications of spatial-temporal shading in agrivoltaics under fixed tilt & tracking bifacial photovoltaic panels

The spatial and temporal behavior of the incident sunlight can have important implications for agrivoltaic (AV) crop yield. Here we explore the short term (daily) and long term (monthly) variations of the photosynthetically active radiation (PAR) under various tracking and fixed-tilt agrivoltaic PV modules configurations and propose strategies to minimize the shade-induced crop yield loss. For a fixed row to row PV module spacing, vertically installed PV modules facing East/West provide the best spatial homogeneity and a higher intensity of net daily incident PAR, while the traditional North/South faced fixed tilt PV modules result in a high spatial contrast, i.e., a significantly lower PAR underneath the PV modules as compared to the open space between the adjacent rows of PV modules. The shading behavior for the horizontal single axis tracking PV modules shows a seasonal dependence, a higher shading vertically below the PV modules during winters and vice versa for summers. The spatial variation in the crop yield correlates well with that of the net daily PAR spatial pattern in the case of the shade sensitive crops (e.g., tomato) while the spatial yield for the shade tolerant crops (e.g., lettuce) is less affected by PAR variations. We propose an intercropping approach where the cultivated areas for crops having high and low shade sensitivity are identified based on the calculated spatial PAR pattern. A case study of intercropping lettuce with tomato under various PV module configurations shows that this approach can minimize the biomass loss for shade sensitive crops in the presence of spatial PAR heterogeneity.

Diurnal and Seasonal Variations of Photosynthetic Energy Conversion Efficiency of Field Grown Wheat.

Improving canopy photosynthetic light use efficiency and energy conversion efficiency (εc) is a major option to increase crop yield potential. However, so far, the diurnal and seasonal variations of canopy light use efficiency (LUE) and εc are largely unknown due to the lack of an efficient method to estimate εc in a high temporal resolution. Here we quantified the dynamic changes of crop canopy LUE and εc during a day and a growing season with the canopy gas exchange method. A response curve of whole-plant carbon dioxide (CO2) flux to incident photosynthetically active radiation (PAR) was further used to calculate εc and LUE at a high temporal resolution. Results show that the LUE of two wheat cultivars with different canopy architectures at five stages varies between 0.01 to about 0.05 mol CO2 mol–1 photon, with the LUE being higher under medium PAR. Throughout the growing season, the εc varies from 0.5 to 3.7% (11–80% of the maximal εc for C3 plants) with incident PAR identified as a major factor controlling variation of εc. The estimated average εc from tillering to grain filling stages was about 2.17%, i.e., 47.2% of the theoretical maximal. The estimated season-averaged radiation use efficiency (RUE) was 1.5–1.7 g MJ–1, which was similar to the estimated RUE based on biomass harvesting. The large variations of LUE and εc imply a great opportunity to improve canopy photosynthesis for greater wheat biomass and yield potential.

Comparative Analysis on the Estimation of Diurnal Solar-Induced Chlorophyll Fluorescence Dynamics for a Subtropical Evergreen Coniferous Forest

Solar-induced chlorophyll fluorescence (SIF) is considered as a prospective indicator of vegetation photosynthetic activity and the ecosystem carbon cycle. The current coarse spatial-temporal resolutions of SIF data from satellite missions and ground measurements still cannot satisfy the corroboration of its correlation with photosynthesis and carbon flux. Practical approaches are needed to be explored for the supplementation of the SIF measurements. In our study, we clarified the diurnal variations of leaf and canopy chlorophyll fluorescence for a subtropical evergreen coniferous forest and evaluated the performance of the canopy chlorophyll concentration (CCC) approach and the backward approach from gross primary production (GPP) for estimating the diurnal variations of canopy SIF by comparing with the Soil Canopy Observation Photosynthesis Energy (SCOPE) model. The results showed that the canopy SIF had similar seasonal and diurnal variations with the incident photosynthetically active radiation (PAR) above the canopy, while the leaf steady-state fluorescence remained stable during the daytime. Neither the CCC nor the raw backward approach from GPP could capture the short temporal dynamics of canopy SIF. However, after improving the backward approach with a correction factor of normalized PAR incident on leaves, the variation of the estimated canopy SIF accounted for more than half of the diurnal variations in the canopy SIF (SIF687: R2 = 0.53, p < 0.001; SIF760: R2 = 0.72, p < 0.001) for the subtropical evergreen coniferous forest without water stress. Drought interfered with the utilization of the improved backward approach because of the decoupling of SIF and GPP due to stomatal closure. This new approach offers new insight into the estimation of diurnal canopy SIF and can help understand the photosynthesis of vegetation for future climate change studies.

Salt Marsh Light Use Efficiency is Driven by Environmental Gradients and Species‐Specific Physiology and Morphology

AbstractLight use efficiency (LUE) of salt marshes has not been well studied but is central to production efficiency models (PEMs) used for estimating gross primary production (GPP). Salt marshes are typically dominated by a species monoculture, resulting in large areas with distinct morphology and physiology. We measured eddy covariance atmospheric CO2 fluxes for two marshes dominated by a different species: Juncus roemerianus in Mississippi and Spartina alterniflora in Georgia. LUE for the Juncus marsh (mean = 0.160 ± 0.004 g C mol−1 photon), reported here for the first time, was on average similar to the Spartina marsh (mean = 0.164 ± 0.003 g C mol−1 photon). However, Juncus LUE had a greater range (0.073–0.49 g C mol−1 photon) and higher variability (15.2%) than the Spartina marsh (range: 0.035–0.36 g C mol−1 photon; variability: 12.7%). We compared the responses of LUE across six environmental gradients. Juncus LUE was predominantly driven by cloudiness, photosynthetically active radiation (PAR), soil temperature, water table, and vapor pressure deficit. Spartina LUE was driven by water table, air temperature, and cloudiness. We also tested how the definition of LUE (incident PAR vs. absorbed PAR) affected the magnitude of LUE and its response. We found LUE estimations using incident PAR underestimated LUE and masked day‐to‐day variability. Our findings suggest that salt marsh LUE parametrization should be species‐specific due to plant morphology and physiology and their geographic context. These findings can be used to improve PEMs for modeling blue carbon productivity.

Intercepted Photosynthetically Active Radiation (PAR) and Spatial and Temporal Distribution of Transmitted PAR under High-Density and Super High-Density Olive Orchards

We quantified the photosynthetically active radiation (PAR) interception in a high-density (HD) and a super high-density (SHD) or hedgerow olive system, by measuring the PAR transmitted under the canopy along transects at increasing distance from the tree rows. Transmitted PAR was measured every minute, then cumulated over the day and the season. The frequencies of the different PAR levels occurring during the day were calculated. SHD intercepted significantly but slightly less overall PAR than HD (0.57 ± 0.002 vs. 0.62 ± 0.03 of the PAR incident above the canopy) but had a much greater spatial variability of transmitted PAR (0.21 under the tree row, up to 0.59 in the alley center), compared to HD (range: 0.34–0.43). This corresponded to greater variability in the frequencies of daily PAR values, with the more shaded positions receiving greater frequencies of low PAR values. The much lower PAR level under the tree row in SHD, compared to any position in HD, implies greater self-shading in lower-canopy layers, despite similar overall interception. Therefore, knowing overall PAR interception does not allow an understanding of differences in PAR distribution on the ground and within the canopy and their possible effects on canopy radiation use efficiency (RUE) and performance, between different architectural systems.

Seasonal Dynamics of Soil Moisture in an Integrated-Crop-Livestock-Forestry System in Central-West Brazil

Integrated-crop-livestock-forestry (ICLF) systems are currently promoted as a measure for sustainable intensification of agricultural production. However, due to complex interactions among ICLF components, we are still lacking evidence about the system’s resilience regarding water availability, especially for regions characterized by pronounced wet and dry seasons and frequent droughts. For a mature ICLF system in the Cerrado biome of central-west Brazil comprising rows of eucalyptus trees (Eucalyptus grandis x Eucalyptus urophylla, H13 clone) at a spacing of 22 m in combination with Brachiaria brizantha cv. BRS Piatã pasture we continuously measured soil moisture (SM) until 1 m depth and supported this data with measurements of photosynthetically active radiation (PAR) and aboveground green grass biomass (AGBM) across transects between the tree rows for almost two years. Across the seasons a distinct gradient was observed with SM being lower close to the tree rows than in the space between them. During winter SM decreased to critical values near the tree lines in the topsoil. During spring and summer, incident PAR was 72% and 86% lower close to the trees than at the center point. For autumn and winter PAR was more evenly distributed between the tree rows due to inclination with notably up to four times more radiation input near the tree lines compared to spring and summer. AGBM showed a clear distribution with maximum values in the center and about half of the biomass close to the tree rows. Our data suggest that, restrictions in AGBM accumulation shifted among seasons between water limitations in winter and light limitations during summer. Interestingly, SM changes during wetting and drying events were most pronounced in subsoils near the tree rows, while the topsoil showed much less fluctuations. The subsoil in central position showed the lowest SM dynamics in response to drought maintaining a relative high and constant SM content, therefore functioning as important water reservoirs likely improving the resilience of the system to drought stress. Results of this study could help to improve management and the design of ICLF systems in view of sustainability and resistance to (water) crises but should be further supported by in depth analysis of soil water dynamics as affected by climate gradients, soil types and different management practices.

Characterizing the three-dimensional spatiotemporal variation of forest photosynthetically active radiation using terrestrial laser scanning data

The three-dimensional (3-D) spatiotemporal distribution of forest canopy photosynthetically active radiation (PAR) is a major determinant of within-crown variations of leaf photosynthesis and transpiration. However, it remains challenging to map the 3-D spatiotemporal distribution of PAR within and under the forest canopy due to the limited ability to capture the detailed 3-D structures of forest canopies.This study proposes a simple approach to estimate the 3-D spatiotemporal distribution of forest canopy PAR at a fine spatial resolution based on terrestrial laser scanner (TLS) data by characterizing difference in transfer of incident solar direct PAR and diffuse PAR and by considering the effect of the orientation angles of foliage elements on the 3-D distribution of direct PAR. Estimated PAR was validated using 10-minute PAR measurements from pyranometers at different locations within four forest plots, at which effective leaf area index (LAIe) ranged from 0.83–4.25. It showed that the proposed method captured 85% (N=2831, p<0.001) of the variations in the field pyranometer measurements. Furthermore, the proposed method was also applicable for estimating the vertical distribution of PAR (R2=0.95, N=15, p<0.001). Direct, diffuse, and total PAR exhibited similar vertical distributions changing with LAIe. It was shown that without differentiation of direct and diffuse PAR or ignoring effects of orientation angles of foliage elements may cause errors of about 20% in the estimated 3-D PAR distribution. Furthermore, the results indicated that the proposed method can be used to estimate the vertical distribution of PAR when the average neighbor point distance (NPD) is ≤20 cm. This proposed method will be usable for accurate separation of sunlit and shaded leaves and estimation of vertical distribution of PAR, which will benefit to reduce uncertainties in the estimation of terrestrial carbon fluxes and retrieval of vegetation structural and biochemical parameters.

Spatial-Temporal Shading Under Mobile &amp; Fixed Tilt Bifacial Agrivoltaic Panels &amp; Implications for the Cropping Practices

Spatial and temporal behavior of the incident sunlight can have important implications for agrivoltaic (AV) crop yield. We explore the daily and monthly variations of the photosynthetically active radiation (PAR) under various tracking and fixed-tilt agrivoltaic solar panels and propose strategies to minimize the shade-induced crop yield loss. Vertically installed solar panels facing East/West provide the best spatial homogeneity and a higher intensity of net daily incident PAR, while North/South faced fixed tilt panels result in a high spatial contrast, i.e., a significantly lower PAR underneath the panels as compared to the spacing between the adjacent panel rows. The shading behavior for the horizontal single axis tracking panels shows a seasonal dependence, a higher shading below the panels during winters and vice versa for summers. The spatial variation in the crop yield correlates well with that of the net daily PAR pattern in the case of the shade sensitive crops (e.g., tomato) while the spatial yield for the shade tolerant crops (e.g., lettuce) is less affected by PAR variations. We propose an intercropping approach using crops with high and low shade sensitivity to minimize the biomass loss related to PAR heterogeneity and present lettuce/tomato intercropping schemes under various agrivoltaic configurations

Performance, radiation capture and use by maize–mungbean–common bean sequential intercropping under different leaf removal and row orientation schemes

AbstractFood security under smallholder farming can be improved through innovative intensification of cropping systems. Maize (Zea mays L.) – mungbean (Vigna radiata (L.) Wilczek) – common bean (Phaseolus vulgaris L.) sequential intercropping was studied to evaluate the patterns of radiation capture and radiation use efficiency and to determine the effects of leaf removal and row orientation on performance and intercropping efficiency. Sequential intercropping captured 1039 MJ m−2 photosynthetically active radiation (PAR) accounting for 70% of incident seasonal PAR. The corresponding sole stands for maize captured 41%, mungbean 29%, common bean 34% and mungbean–common bean 63%. Intercropped components had interception ratios of 0.98, 0.31 and 0.61 for maize, mungbean and common bean, respectively. Associated maize used intercepted light with similar efficiency, mungbean with greater efficiency and common bean with lesser efficiency compared to sole crops. Maize leaf removal and row orientation had no significant effect on performance and partial land equivalent ratio (LER) of maize. Leaf removal under East–West (EW) orientation increased grain yield by 96%, total biomass by 63%, partial LER by 92%, in common bean and total LER by 7%. Leaf removal also improved grain yield, biomass yield, partial LER, in common bean and total LER during the wetter year of 2013. Similarly, EW orientation was advantageous in 2013 raising total LER by 8%. Maize leaf removal and EW row orientation had synergistic effects on intercropping efficiency and economic benefit and both have exerted positive influence under favourable weather. Total LER values of 1.47 in 2013 and 1.29 in 2015 had revealed greater biological efficiency for intercropping during both years though it was more profitable in 2013. Thus, the cropping system can be adopted under timely onset of the rainy season using EW row orientation while leaf removal can also be practiced depending on weather conditions and convenience.

Tree species effects on understory forage productivity and microclimate in a silvopasture of the Southeastern USA

Ecosystem services provided by silvopastoral systems are mediated by specific management practices, environmental conditions, and overall design of the system. We hypothesized that selection of tree species affects understory forage nutritive value and productivity, light/shade environment, and microclimate. The silvopastoral system was located at the Center for Environmental Farming Systems in Goldsboro, North Carolina, USA. Three overstory tree-species were Pinus palustris (PP; longleaf pine), Pinus taeda (PT; lobloblly pine), and Quercus pagoda (QP; cherrybark oak). The understory forage component consisted of a four-way mixture of native warm-season grasses [big bluestem (Andropogon gerardii, ‘Eastern’, KY origin), eastern gamagrass (Tripsacum dactyloides, MO origin), indiangrass (Sorghastrum nutans, ‘NC ecotype’), and switchgrass (Panicum virgatum, ‘Alamo’)]. The experimental design was an RCBD with 3 replicates. There was no effect of seedbed preparation (till versus no-till) on forage establishment. Understory dry matter yield, crude protein and total digestible nutrient concentrations of the harvested forage were not affected by tree species, with the exception at the 3.5 south sampling point. Overstory effects on microclimate variables were not different among tree-species, but were more noticeable during the daytime of the summer months, and were at the most 1-degree point for temperature and temperature-humidity index and 3 points for relative humidity. The silvopasture design in our study provided year-round shade by the tree-component, with varying levels of shade (ranging from 90 to 6% of incident photosynthetic active radiation) due to geographic location, tree species, and season. Our results describe and highlight the potential of trees in a silvopasture design in the southeastern USA to mitigate changes in temperature, humidity, the temperature-humidity index, and forage productivity and as a function of tree species and at different distance from the trees.

Seasonal Variations of Marine Environment and Primary Production in the Taiwan Strait

The first data set of seasonal marine environment and euphotic zone integrated primary production (IP) variations in the Taiwan Strait was reported. The measured annual IP was 123±86 gC m-2 y-1 (338±235 mgC m-2 d-1), and its seasonal variations can be described with a left-skewed normal distribution curve. The average seasonal IP values from the highest to the lowest were summer (664±270 mgC m-2 d-1), autumn (350±118 mgC m-2 d-1), spring (202±110 mgC m-2 d-1) and winter (137±68 mgC m-2 d-1). The lowest IP was during the nutrient-rich winter because it had a short insolation duration, low incident photosynthetic active radiation (PAR) and low light transmission (shallow euphotic zone depth) due to strong vertical mixing. In contrast to the winter, the highest IP was during the nutrient-depleted summer, which had a long insolation duration, high incident PAR and high light transmission (deeper euphotic zone depth). In addition, the heterotrophic nutrients from upwelling in the south might also support the highest IP in summer. As three primary water masses exist in the Taiwan Strait and three of them have different characteristics, different mixing ratios of water masses may cause different chemical and hydrographic conditions, which leads to different levels of Chl a concentrations and primary production. It is worth to mention that offshore wind farm (OWF) construction in the Changyun Rise (CYR) of the Taiwan Strait is on-going. As primary production is the foundation for a marine ecosystem and supports the food web and fish stock, the results of this research can not only be used as the baseline for evaluating the OWF impact on the marine ecosystem but also be used for assessing their influence on fishery resources.

Fluorescence Correction Vegetation Index (FCVI): A physically based reflectance index to separate physiological and non-physiological information in far-red sun-induced chlorophyll fluorescence

Sun-induced chlorophyll fluorescence (SIF) has been used to track vegetation photosynthetic activity for improving estimation of gross primary productivity (GPP) and detecting plant stress. There are both physical and physiological controls of SIF measured at the surface and retrieved from remote sensing including satellite observations. In order to accurately use SIF for monitoring of plant physiology, the effects of physically-based radiation processes related to leaf and canopy structure, notably photosynthetically active radiation (PAR) absorption and SIF scattering and re-absorption, must be characterized. In this study, we investigate both PAR absorption and SIF scattering processes and find that although it is difficult to quantify their effects individually by using just reflectance, the combined effects of the two processes can be well approximated by a reflectance index. This index, referred to as FCVI (Fluorescence Correction Vegetation Index), is defined as the difference between near-infrared (NIR) and broad-band visible (VIS, 400–700 nm) reflectance acquired under identical sun-canopy-observer geometry of the SIF measurements. The development of the index was based on the physical connection between reflectance and far-red SIF, which was revealed by using the spectral invariant theory. The utility of FCVI to correct far-red SIF for PAR absorption and scattering effects, thus improving the link to photosynthesis, was tested with data from: (i) a field experiment for a growing season; and (ii) a numerical experiment which included a number of scenarios generated by a radiative transfer model. For both the observations and simulations, the FCVI provided a promising estimate of the impact of the physically-based radiation processes on far-red SIF of moderately dense canopies (i.e., FCVI ≥ 0.18). Normalizing the TOC far-red SIF by both the incident PAR (iPAR) and the FCVI provided a good estimate of the far-red fluorescence emission efficiency of the canopies examined. This approach enhances our ability to generalize retrievals for vegetation processes as they change through natural growth phases and seasons. Taken together, far-red SIF and FCVI may enable the assessment of the light partitioning of vegetation canopies, an essential step to facilitate the use of far-red SIF for tracking physiological processes.

Leaf and Crown Optical Properties of Five Early-, Mid- and Late-Successional Temperate Tree Species and Their Relation to Sapling Light Demand

The optical properties of leaves and canopies determine the availability of radiation for photosynthesis and the penetration of light through tree canopies. How leaf absorptance, reflectance and transmittance and radiation transmission through tree canopies change with forest succession is not well understood. We measured the leaf optical properties in the photosynthetically active radiation (PAR) range of five Central European early-, mid- and late-successional temperate broadleaf tree species and studied the minimum light demand of the lowermost shade leaves and of the species’ offspring. Leaf absorptance in the 350–720 nm range varied between c. 70% and 77% in the crown of all five species with only a minor variation from the sun to the shade crown and between species. However, specific absorptance (absorptance normalized by mass per leaf area) increased about threefold from sun to shade leaves with decreasing leaf mass area (LMA) in the late-successional species (Carpinus betulus L., Tilia cordata Mill., Fagus sylvatica L.), while it was generally lower in the early- to mid-successional species (Betula pendula Roth, Quercus petraea (Matt.)Liebl.), where it changed only a little from sun to shade crown. Due to a significant increase in leaf area index, canopy PAR transmittance to the forest floor decreased from early- to late-successional species from ~15% to 1%–3% of incident PAR, linked to a decrease in the minimum light demand of the lowermost shade leaves (from ~20 to 1%–2%) and of the species’ saplings (from ~20 to 3%–4%). The median light intensity on the forest floor under a closed canopy was in all species lower than the saplings’ minimum light demand. We conclude that the optical properties of the sun leaves are very similar among early-, mid- and late-successional tree species, while the shade leaves of these groups differ not only morphologically, but also in terms of the resource investment needed to achieve high PAR absorptance.

From sunlight to seed: Assessing limits to solar radiation capture and conversion in agro-ecosystems

Yield depends upon the amount of photosynthetically active radiation (PAR) absorbed by the crop during the growing season and its conversion into harvestable biomass. Little is known about attainable and actual efficiencies involved in absorbing incident PAR (ea) and converting it into yield (ec) in producer fields. We developed a novel approach consisting of producer data, satellite imagery, and crop modelling to assess ea and ec in agro-ecosystems. Simulated phenology and satellite imagery were used to estimate incident (IPAR) and absorbed (APAR) PAR during the crop season for 3096 soybean fields sown across the US North Central region during 2014–2016. Quantile regression was used to derive upper limits for ea and ec and multiple-regression analysis was performed to identify biophysical drivers of the observed variation in these two efficiencies. Differences in weather, soil, and management resulted in a wide range in crop cycle length (58–126 d), total IPAR (580–1250 MJ m−2), and seed yield (0.8–6.3 Mg ha−1). The relationship between yield and total APAR was curvilinear, indicating marginal yield gains in the upper APAR range. Attainable ea represented 65% of total IPAR, while attainable ec ranged from 0.6 to 1.2 g seed MJ−1 depending upon total APAR. Average producer ea and ec were 14 and 29% below their attainable efficiencies. Average efficiency in converting IPAR into yield was 0.8%, with an upper limit of 1.1% derived from the 95th percentile of the field data distribution. Although weak, relationships between efficiencies and meteorological factors such as IPAR, temperature, water balance, and diffuse radiation were consistent with previous literature. There was a strong trade-off between ec and ea indicating that maximizing both efficiencies simultaneously was not possible in producer fields. Our approach can be used to determine attainable and actual efficiencies in capturing and converting radiation into yield, set realistic yield limits, and understand relationships between producer yield and management practices.

Effect of Orchard Management Factors on Flesh Color of Two Red-Fleshed Apple Clones

Little is known about factors affecting anthocyanin biosynthesis in red-fleshed apples (Malus × domestica Borkh.). The objective was to establish the effects of orchard management factors on flesh anthocyanin content of dark-colored (DC) and light-colored (LC) apple clones. Flesh color was assessed by measuring color in the L, a, b mode using a spectrophotometer and predicting the anthocyanin content based on relationships between the absorption of a flesh extract at 530 nm and the L-value determined using a spectrophotometer (r2 = 0.99 ***). Fruit from the DC clone were red by 86 days after full bloom (DAFB), whereas the LC clone began to color at 136 DAFB. Color intensity in both clones decreased from the top of the tree to the base. Further, the intensity of the flesh color of the DC clone decreased with shading (94% absorption of incident photosynthetic active radiation). Covering a fruit with a UV absorbing film (100% UV absorption) had no effect on flesh color in the DC clone but decreased color in the LC clone. Fruit thinning increased color in DC and LC fruit. There was little change in flesh color during storage. However, the DC clone developed severe flesh browning as storage progressed beyond 30 days. The results demonstrated that light (visible and UV wavelength) stimulated, whereas shade inhibited, anthocyanin biosynthesis in the flesh under orchard conditions.